In the batch pulping process, wood chips and cooking liquor are loaded into a digester which is then closed. The pressure and the temperature within the digester are then raised by admitting steam to the digester to cook the chips whereby the chemicals of the cooking liquor react with the lignin which binds the fibers together and the lignin is dissolved. At the end of the cooking step, the content of the digester is, using a prior art method, forced out from the digester to a blow tank at near atmospheric pressure. Due to the sudden pressure drop, the hot liquor within the chips vaporizes tending to break up the chips into individual fibers. A large amount of steam is released during the blow period. The steam is condensed by direct contact with cooled condensate and the condensate is collected in an accumulator to be used as a heat medium in auxiliary portions of the pulping process. The pulp and the spent cooking liquor is passed to pulp washers where the liquor is washed from the pulp. The energy recovery rate of this system is relatively low.
It is known that discharging of the pulp from the digester at the cooking temperature, i.e. about 170.degree. C., tends to cause damage to the fibers and to lower the pulp strength. It has therefore been necessary to lower the temperature of the pulp before it is blown out of the digester. This method, known as Cold Blow, is accomplished by introducing liquor from the pulp washing section at a temperature of 70.degree.-90.degree. C. to the bottom of the digester. The temperature in the blow line of the digester will be about 5.degree. C. higher than the temperature of the liquor added. The systems in which cooling of the pulp is brought about in the digester are quite complicated. To install a cold blow system into an existing digester requires extensive rebuilding of the digester and a long shutdown time.
U.S. Pat. No. 3,567,573 discloses a process in which the cooked wood chips together with the hot cooking liquor is discharged from a digester to a vessel at a pressure significantly lower than that to which it is exposed in the digester. Through the reduction in pressure, the chips are defibered by expansion of the liquid in the chips. The resulting pulp is washed and cooled by a weak and relatively cool black wash liquor which is introduced through nozzles disposed in the lower part of the vessel. The wash liquor is caused to flow upwardly through the vessel and counter to the flow of the hot pulp-liquor mass. A relatively hot and stronger black wash liquor flows out through screens located in the upper part of the vessel. The cooled and washed pulp is withdrawn in a continuous flow. The major portion of the heat present in the chip and liquor mass is converted into steam.
It is accordingly an object of the present invention to provide an economical cold blow method by which heat can be recovered at a high energy recovery rate in batch type pulp digesting systems.
It is another object of the present invention to provide a simple cold blow system which is easy to install in existing pulp digesting systems.
A further object of the present invention is to provide a cold blow system which is easy to operate and control.
Whereas prior art systems recover heat as steam, it is one of the characteristic features of the present invention that the pulp together with the spent cooking liquor is discharged from the digester substantially without any steam generation, i.e. that the major portion of the heat is recovered as sensible heat in a hot liquor stream.